The present invention relates to a fuel supply system for an internal combustion engine. The fuel supply system switches a fuel supplying path to an engine so as to switch fuel injection pressure to the engine.
Recent fuel supplying apparatuses are required to supply greater amount of fuel to a combustion chamber to deal with the demands for increased power of engines and the use of alcohol-containing fuel. The stoichiometric air-fuel ratio of the alcohol-containing fuel is smaller than the stoichiometric air-fuel ratio of gasoline. The injection amount of fuel from an injector is changed based on an open period of the injector and fuel injection pressure. The fuel injection pressure is fuel pressure in a delivery pipe that supplies fuel to the injector. In basic control for controlling the fuel injection amount, the open period of the injector is controlled. However, in case of intake stroke injection, a maximum value of the open period of the injector is limited by the period of the intake stroke. In the intake stroke injection, air-fuel mixture is generated by injecting fuel from the injector during the intake stroke. This inevitably limits the open period of the injector. Therefore, it is difficult to supply a great amount of fuel to the combustion chamber only by extending the open period of the injector.
As the fuel injection pressure is set to be higher, the fuel injection amount per unit time is increased. Therefore, it is predicted that the fuel injection amount can be greatly increased in the intake stroke injection.
However, in an actual operation, if the fuel injection pressure is set to be high, the change amount of the fuel injection amount with respect to the change of the open period of the injector is increased. Therefore, precise control of the fuel injection amount may be difficult only by setting the fuel injection pressure to be high. This is because there is a limit to reduction of the change amount of the open period of the injector. That is, control resolution of the injector cannot be set to be smaller than a predetermined value. For example, Japanese Laid-Open Patent Publication No. 5-59976 discloses a high-pressure return passage and a low-pressure return passage that return excess fuel from the delivery pipe to the fuel tank. The high-pressure return passage has a high-pressure regulator and the low-pressure return passage has a low-pressure regulator.